Spherical abberation and chromatic abberation are totally different beasts. you are correct that CA is caused by variations in the refractive index of glass with wavelength. Effectively CA means that the focal length of a lens is slightly different for different wavelengths. Colour fringing Effects are most pronouced at the edges of the frame because each colour gets a slightly different magnification, and the difference builds up from the centre of the image out. Stopping down does not (in theory) reduce CA.
Spherical abberation occurs because the ideal image forming surface is not a sphere but a hyperbola. Spheres are just easier to make, and are close enough. Sperical abberation can be reduced by stopping down the lens. (because a sphere is closest to a hyperbola in the middle) -Scott On Sat, 2004-05-22 at 12:56, Jens Bladt wrote: > Joe > CA is (AFAIK) caused by the fact, that glas does diffract different colors > (wave lengths) differently (like a prisma). This means in practice, that > it's not possible for all colours in a photograph to be equally sharp in the > same surface film-plane. This is why they invented the (expensive) > APO-lenses, that are not sfaerical (parts of a ball), but asfaerical and > especilally made for avoiding/minimizing Chromatic Aberations. All this > means, that it's not just the fringes that causes problems - it's all over > the picture. But it's indeed very notisable near contours/edges. > All the best. > Jens > > mailto:[EMAIL PROTECTED] > http://hjem.get2net.dk/bladt > > > -----Oprindelig meddelelse----- > Fra: Joseph Tainter [mailto:[EMAIL PROTECTED] > Sendt: 22. maj 2004 18:25 > Til: pdml > Emne: Today's Question #1: CA > > > How does chromatic aberration show other than as color fringing near the > edges? Or are they the same thing? > > Thanks, > > Joe > > >
